1. Field of the invention
The present invention relates, generally, to a compact steam reformer and, in particular, to a compact steam reformer in which a reforming reactor, a high- and a low-temperature converter, a heat exchanger, and a steam generator are integrated into a module, whereby hydrogen is generated at low cost.
2. Description of the Prior Art
On the whole, synthetic gas is obtained from natural gas or naphtha by the steam reformation on catalysts. A typical reforming process is exemplified by the following Reaction Equations 1 and 2:CH4+H2O→CO+3H2ΔH°298=206.1 kJ/mol   [Eq. 1]CO+H2O→CO2+H2ΔH°298=−41.16 kJ/mol   [Eq. 2]
Natural gas is first reacted with excess steam on a catalyst, for example, Ni/Al2O3 (Eq. 1) and then subjected to conversion reaction (e.g. hydrogenesis reaction, hydrogen conversion) (Eq. 2). According to final uses of the product gas, its equilibrium composition is determined by the conversion reaction which is carried out in two steps at high and low temperatures.
As denoted in Reaction Equation 1, the steam reforming reaction is highly exothermic. Depending on the next process, the operation conditions of the steam reforming reaction are determined within the pressure range of atmospheric pressure to 30 atm, and the temperature range of 700 to 900° C. In this reaction, excess steam is needed in order to prevent the deposition of coke on the catalyst. Typically, the mole ratio of H2O/CH4 is in the range of 2 to 6, depending on purposes of the product gas. In most cases, Ni-based catalysts are employed while K compounds i. e. K2O, Ca, or MgO, serve as co-catalysts in order to restrain the deposition of coke. In the primary reforming reaction, generally, methane is converted at a ratio of 90–92% with concurrent production of carbon monoxide and hydrogen in addition to carbon dioxide and water. The closer to 3:1 the molar ratio between produced hydrogen and carbon monoxide is, the more ideal the reaction is.
Next to the primary steam reforming reaction, a secondary reaction may be conducted in which unreacted methane is removed through reaction with oxygen. In addition, there have been suggested processes that use combinations of methane, carbon dioxide and oxygen. The product gas obtained in the reforming reaction undergoes the two-step hydrogen gas conversion of high and low temperatures as shown in Reaction Equation 2, so that carbon monoxide is oxidized to carbon dioxide.
In the first step of the hydrogen gas conversion reaction, the oxidation of carbon monoxide to carbon dioxide occurs on a Fe catalyst at 330–530° C., while the second step is carried out at 200–260° C. in the presence of a Cu catalyst to react carbon monoxide with water to produce carbon dioxide and hydrogen. After passing through a conversion reactor in which most carbon monoxide is removed, the gas mixture contains hydrogen and carbon, along with trace amounts of unreacted methane and carbon monoxide. All or most of the carbon dioxide is removed through a pressure swing adsorption (PSA) process with concurrent yield of hydrogen of 99.9 vol % or higher.
With reference to FIG. 1, there is a conventional process flow of a steam reforming method for naphtha. As seen in this figure, the steam reformer system is a combination of independent modules, including a steam generator, reformers, a high temperature converter, a heat exchanger, and a low temperature reformer, and thus its structure is complex and difficult to make compact. Additionally, a flame burner type combustor to provide the heat required for the reforming reaction in the conventional reformer system occupies too large a space and cannot maintain a uniform temperature distribution in the reformer.
When applying the structure of FIG. 1 to an ICI method, which is suited for the generation of hydrogen for ammonia synthesis from naphtha, the steam reforming system of FIG. 1 is composed of a desulfurizer 1, a steam generator 2, primary and secondary reformers 3, and high and low temperature converters 4 and 5.
After being mixed with the steam produced from the steam generator 2 using a gas turbine arrangement or a separate steam boiler, naphtha which was previously deprived of sulfur in the desulfurizer 1 is passed through the primary reformer filled with nickel based catalysts to produce decomposed gases consisting mainly of hydrogen, carbon dioxide, carbon monoxide and methane. Owing to the endothermic nature of this decomposition reaction, the temperature of the reformer is maintained at 800 to 850° C. by use of a flame burner or a turbine arrangement. As for the reaction pressure, it is controlled according to the pressure required in the subsequent process. In the case of the generation of hydrogen for production of ammonia, the process is usually conducted at around 30 atm.
Then, the decomposed gases are deprived of methane in the secondary reformer 3. In this regard, pure oxygen is fed to the decomposed gases to partially oxidize methane into carbon monoxide and hydrogen. For additional oxidation of carbon monoxide into carbon dioxide, it is reacted with steam in the high- and low-temperature converters 4 and 5 with concurrent production of hydrogen. Thereafter, effluents from the converters 4 and 5 are transferred to an adsorption column or subjected to a PSA process in which the carbon dioxide of the effluents is utilized for production of potassium carbonate while hydrogen is obtained at a purity of 99.9 vol % or higher. Afterwards, for ultra purification of the hydrogen gas, the carbon monoxide that exists at a trace amount is converted into methane and water by reaction with hydrogen in a methanation reactor. The resulting hydrogen gas of ultra purity is compressed to 250 to 300 kg/cm2 and transferred to an ammonia synthesis apparatus.
However, the conventional system for synthesizing hydrogen using a steam reforming reaction, like the ICI method, is too voluminous because it is composed of separate modules, such as a steam generator, a reformer, and a converter. Additionally, insulated pipes must be laid between the modules to communicate with each other for gas transfer. On the whole, the structure of the system is complex. Most of the reformers used in conventional systems are structured to have flame burner type combustors established at lower or upper parts thereof. However, such a combustor is unable to provide heat uniformly over the reactor.
U.S. Pat. No. 5,733,347 discloses a fuel gas reformer structure formed from flat plate reformer components. The fuel gas reformer structure itself is compact, but it requires the above-mentioned reactors additionally, so that the whole structure is not compact. Because the fuel gas reformer structure is heated from both sides by flanking burners, its temperature is elevated to a desired value within such a short period of time as to increase the efficiency of the fuel gas reformation. However, this efficiency is poorer than that of the structure having a burner surrounding a reformer.
A compact, multi-fuel steam reformer can be found in U.S. Pat. No. 5,938,800. This reformer, however, employs a flame burner and suffers from the disadvantage similar to that of U.S. Pat. No. 5,733,347.
PCT WO 98/08771 introduces a cylindrical reforming apparatus, which reforms natural gas by a combination of partial oxidation of methane and steam reformation to produce hydrogen and carbon dioxide, in which a reforming catalyst portion, a spiral pipe, and a converter are integrated. In the converter, the heat generated from the partial oxidation of natural gas with oxygen is utilized for steam reforming. The integration ensures the compactness of the structure to some extent, but the structure requires an additional apparatus, apart from the reformer, for generating steam outside and providing it to the reformer. Further, a separate oxygen generator is provided because pure oxygen is needed for the partial oxidation. Accordingly, much difficulty is found in setting up the structural modules in terms of time, cost and arrangement design. In addition, the reforming system is large in total volume. Requiring no oxygen generators, an alternative is suggested to use air instead of pure oxygen. In this case, however, the synthesized gas does not exceed a purity of 70 vol %, unlike that synthesized in the steam reforming reaction, owing to the presence of nitrogen in air.
Korean Pat. No. 246079 discloses a method for producing hydrogen and carbon monoxide from methane, in which the carbon dioxide and steam generated as the autoexhaust of methane is recovered to a rector, and used as materials, together with excess methane. This method enjoys the advantage of eliminating both a steam generator and a burner for providing a reaction heat because the recovered effluent contains both steam and heat. However, the reactor is provided installed, aside from the reformer. Accordingly, this method cannot make the reforming apparatus structure compact in total, therefore not solving the problems mentioned above.
Korean Pat. No. 156088 discloses a fuel reforming apparatus that uses methanol as a material for producing hydrogen-rich gas with a CO content less than 1% as well as a fuel for a burner for providing the heat necessary for the hydrogen production. In the fuel reforming apparatus, a single circular catalyst pipe is positioned at the center while a methanol burner is provided at a lower position. The heat generated from the burner is transferred in two directions of the inside and outside of the catalyst pipe to improve heat efficiency and make the catalyst bed of the pipe have a small temperature gradient. The circular type apparatus is certainly improved in thermal efficiency, but suffers from the problems caused by use of a flame burner in addition to being not compact on the whole.
Korean Utility No. 185299 refers to a burner for gas boilers, which comprises a burner plate in which a plurality of flame holes are provided, a damper plate for maintaining a constant fuel gas flow, a metal fiber layer formed of porous metal fiber or a ceramic fiber layer therebetween to stabilize flames. This burner is described to prevent flame-floating phenomenon and backfiring and to be low in noise level during combustion. However, the use of this burner is limited to hot water boilers in which the burner is used in an updraft. In addition, when a burner is used in such an outward manner that heat is emitted outwardly, a heat provision is difficult to provide to a reformer relative to when a burner is used in an inward manner that heat is focused inwardly. Further, where a burner is provided to a portion of an apparatus, a large temperature gradient may occur if heating begins from a proximal site of the location of the burner.
Korean Utility Laid-Open Pub. No. 2000-8520 relates to a burner formed of metal fibers. This burner, which is used in an outward manner for providing heat in a downdraft to a boiler, also suffers from the same problems as mentioned above.